1. Field of the Invention
The present invention relates to steel wires and steel rods used in a manufacture of various components such as bolts and shafts, which have relatively high strengths, and more particularly to quenched and tempered steel wires with excellent cold forging properties, which can be produced by maintaining a new parameter relating to material quality affecting cold forging properties of the steel wires within a specific range, without additional heat treatment such as quenching and tempering.
2. Description of the Prior Art
In general, components for use in machine structures with relatively high strength, such as hexagon head bolts, U-shaped bolts, ball studs, and shafts, are produced by subjecting steel wires or steel rods (referred to as xe2x80x9csteel wiresxe2x80x9d hereinafter) to cold forging procedures. Such components for use in machine structures are produced in such a way that steel wires are heated at a temperature of 700xc2x0 C. for a period over ten hours so that structures of the steel wires are spheroidized to improve cold forging properties, as in a process indicated bellow.
Steel wire or steel rodxe2x86x92spheroidizing annealing for a long timexe2x86x92cold forgingxe2x86x92heating at a high temperature (850xc2x0 C. or more)xe2x86x92quenching (water or oil)xe2x86x92temperingxe2x86x92product
As will be appreciated from the above, the steel wire or steel rod is necessarily subjected to heat treatment such as quenching and tempering to enhance its strength and toughness even after the cold forging, and it is necessary to perform a plurality of production procedures due to its complicated production process.
Therefore, the conventional process as described above has problems as follows, and is required to be improved in energy efficiency, productivity and working conditions.
1) Since steel wires must be subjected to spheroidizing annealing for a long time, loss of heat energy is increased and productivity is decreased.
2) Since worked steel wires are required to be additionally subjected to quenching and tempering to enhance strength and toughness of the worked steel wires in a manufacturing process, its production time is increased. In addition, working conditions are deteriorated where the worked steel wires are subjected to heat treatment in a manufacturing place. Where the heat treatment is subcontracted to an outside manufacturer, cost for heat treatment and labor for managing delivery schedules are increased, thereby complicating overall process management.
3) Owing to the problems disclosed in above items 1) and 2), reduced productivity is caused due to a heat treatment process. Therefore, there exists an urgent need to improve productivity.
As described above, improvements in productivity, manufacturing cost, working conditions and the like related to the heat treatment are actively demanded.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide quenched and tempered steel wires with excellent cold forging properties, which can be produced without additional heat treatment such as quenching or tempering by performing the heat treatment prior to cold forging.
In order to accomplish the above object, the present invention provides a steel wire having quenched and tempered structure prior to a cold forging process, wherein a product (nxc3x97YS) of a yield strength (YS) and a work hardening coefficient (n), obtained by a tensile test performed with respect to the steel wire, is within a range of 4.0-11.0 kgf/mm2.
The present invention also provides a steel wire produced by drawing the above steel wire, wherein a product (nxc3x97YS) of a yield strength (YS) and a work hardening coefficient (n), obtained by a tensile test performed with respect to the drawn steel wire, is within a range of 1.5-8.5 kgf/mm2.